1. Technical Field
The present invention relates to a color conversion device, which is input with three input signals representing colors and performs conversion into set colors from the input signals, and an image display apparatus having the color conversion device.
2. Related Art
Some of the known color printers, color copying machines, color televisions, or color projectors includes a color conversion device using a three-dimensional lookup table (hereinafter, referred to as ‘3D-LUT’) in order to realize the accurate color reproduction with respect to an image corresponding to an input signal. In general, R, G, and B signals as input signals are expressed as 8-bit or more gray-scale data, that is, gray-scale levels corresponding to 256 or more gray scales. Accordingly, the 3D-LUT is requested to store (256×256×256) or more output values (output color values), in correspondence with all combinations of gray scales of the R, G, and B signals. For this reason, the 3D-LUT needs an extremely large memory capacity.
Therefore, in an actual 3D-LUT, a necessary memory capacity is reduced by using a configuration in which output color values corresponding to combinations of approximate gray scales obtained by dividing gray scales of each of the R, G, and B signals with proper distances, for example, combinations of gray scales of the R, G, and B signals corresponding to upper three or four bits are stored. In addition, on the basis of a lower bit portion excluding the upper three or four bits, an interpolation operation on the output color values obtained from the 3D-LUT is performed (for example, refer to JP-A-58-16180).
An example of an interpolation method disclosed in JP-A-58-16180 is expressed by using FIG. 4 and the following expression A.
                              U          ⁡                      (                          x              ,              y              ,              z                        )                          =                                            U              ⁡                              (                                                      x                    i                                    ,                                      y                    i                                    ,                                      z                    i                                                  )                                      ·                          (                              1                -                                  x                  f                                            )                                +                                    U              ⁡                              (                                                                            x                      i                                        +                    1                                    ,                                      y                    i                                    ,                                      z                    i                                                  )                                      ·                          (                                                x                  f                                -                                  y                  f                                            )                                +                                    U              ⁡                              (                                                                            x                      i                                        +                    1                                    ,                                                            y                      i                                        +                    1                                    ,                                      z                    i                                                  )                                      ·                          (                                                y                  f                                -                                  z                  f                                            )                                +                                    U              ⁡                              (                                                                            x                      i                                        +                    1                                    ,                                                            y                      i                                        +                    1                                    ,                                                            z                      i                                        +                    1                                                  )                                      ·                          z              f                                                          (        A        )            
FIG. 4 illustrates a method of dividing a cube region, and a cube region to be interpolated is divided on the basis of three planes of x=y, y=z, and z=x. As a result, the cube region is divided into six tetrahedrons T1 to T6. The expression A indicates an interpolation method in the second tetrahedron T2. Here, xi, yi, zi refers to an upper bit portion of an input, xf, yf, zf refers to a lower bit portion of the input, and U(x, y, z) refers to an output color value in an input (x, y, z). The length of a side of the cube region is ‘1’. In the interpolation method, the size relation in a lower bit portion indicating a point to be interpolated is examined, it is determined whether the point to be interpolated is included in which tetrahedron, output color values corresponding to four apices of the corresponding tetrahedron are called from a color correction memory, and the output color values are multiplied by four coefficients obtained by subtraction of the lower bit portion and then respective results are added.
In the known technique described above, four multipliers are needed, as can be seen from the expression A. Since the circuit size of a multiplier is large, it is required to decrease the number of multipliers in order to make a device small. Accordingly, it may be considered to decrease the number of multipliers by changing the expression A to expression B.
                              U          ⁡                      (                          x              ,              y              ,              z                        )                          =                              U            ⁡                          (                                                x                  i                                ,                                  y                  i                                ,                                  z                  i                                            )                                +                                    [                                                U                  ⁡                                      (                                                                                            x                          i                                                +                        1                                            ,                                              y                        i                                            ,                                              z                        i                                                              )                                                  -                                  U                  ⁡                                      (                                                                  x                        i                                            ,                                              y                        i                                            ,                                              z                        i                                                              )                                                              ]                        ·                          x              f                                +                                    [                                                U                  ⁡                                      (                                                                                            x                          i                                                +                        1                                            ,                                                                        y                          i                                                +                        1                                            ,                                              z                        i                                                              )                                                  -                                  U                  ⁡                                      (                                                                                            x                          i                                                +                        1                                            ,                                              y                        i                                            ,                                              z                        i                                                              )                                                              ]                        ·                          y              f                                +                                                                 [                                                      U                    ⁡                                          (                                                                                                    x                            i                                                    +                          1                                                ,                                                                              y                            i                                                    +                          1                                                ,                                                                              z                            i                                                    +                          1                                                                    )                                                        -                                      U                    ⁡                                          (                                                                                                    x                            i                                                    +                          1                                                ,                                                                              y                            i                                                    +                          1                                                ,                                                  z                          i                                                                    )                                                                      ]                            ·                              z                f                                                                        (        B        )            
In a color conversion device designed according to the expression B, it is possible to make a device small. However, a problem that the gray balance is easily adversely affected has occurred in the following reasons.
As for colors on a diagonal axis in the cube region, R=G=B is satisfied. Accordingly, the colors on the diagonal axis correspond to colors on a gray axis in a color space. For this reason, in the case when color conversion into a value deviating from the gray axis is made even though color conversion into a color on the gray axis should be actually made, the difference is easily determined by human eyes. In contrast, in the color conversion device designed according to the expression B, even when input signals are values on a gray axis on which R=G=B, that is, xf=yf=zf are satisfied, the conversion into colors other than a gray color is easily made if quantization errors of respective multipliers are accumulated. That is, there has been a problem in that the gray balance is easily adversely affected.